Device for distributed maximum power tracking for solar arrays

ABSTRACT

A system for providing power from solar cells whereby each cell or cell array is allowed to produce its maximum available power and converted by an operatively connected DC/DC converter. Each cell or cell array has its own DC/DC converter. In one form the system includes one or more solar generators wherein each solar generator has one to nine solar cells; a maximum power tracker operatively associated with each solar generator, each maximum power tracker including a buck type DC/DC converter without an output inductor, each maximum power tracker being operatively connected in series with each other; an inductor operatively connected to the series connected maximum power trackers; and means for providing electrical power from the inductor to load means, wherein each maximum power tracker is controlled so that the operatively associated solar generator operates at its maximum power point to extract maximum available power.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 12/953,337, filed Nov. 23, 2010; which is acontinuation application of U.S. patent application Ser. No. 11/571,603,filed Feb. 1, 2007, now U.S. Pat. No. 7,839,022; which is a nationalphase application of International Application Ser. No.PCT/AU2005/001017, filed Jul. 12, 2005; which claims the benefit ofAustralian Provisional Patent Application Ser. No. 2004903833, filedJul. 13, 2004, the disclosures of which applications are incorporatedherein by reference.

FIELD OF INVENTION

The present invention relates to solar cell technology and in particularto maximum power tracking converters. The present invention hasparticular but not exclusive application for use in vehicles that are atleast in part electrically powered by solar cells. Reference to solarpowered vehicles is by means of example only and the present inventionhas application in other areas.

BACKGROUND OF THE INVENTION

A solar cell is a device able to convert incident light to electricalpower. Many solar cells are typically grouped to form an array of solarcells. To collect the electrical power from the solar cells, groups ofcells are either directly connected in series or in parallel. Where thecells are connected in series, they must have identical currents but ifthe cells are connected in parallel they must operate with identicalvoltages. An individual cell will produce maximum power at a unique cellvoltage and current which will vary from cell to cell. The combinationof voltage and current that allows a cell to produce its maximum poweris termed the maximum power point. The maximum power point varies withcell illumination and temperature. Connection of the cells in seriesforces cells to have identical current while connection in parallelforces cells to have identical voltage. Direct connection in series orparallel results in failure to collect all the available electricalpower from the solar cells in the array and at least some of the cellswill operate at a condition other than at their maximum power points.

To obtain the maximum available power from a group of solar cellsconnected in an array or sub-array, a maximum power tracking device isused. Maximum power tracking devices are DC to DC power converters thatallow an array or sub-array to operate at their maximum power point. ADC to DC converter can transform a power input at a certain voltage andcurrent to be transformed to a DC power output at a differing voltageand current. A key feature of all maximum power trackers is a controldevice that determines the point of maximum power for the connectedsolar cells and acts to adjust the DC to DC converter performance toadjust the cell voltage or current to extract the maximum availablepower.

However there are a number of problems or disadvantages associated withthe use of a single maximum power device to control the voltage orcurrent of an array or sub-array of solar cells.

Where solar cells are used to power vehicles, the vehicles are usuallyaerodynamically designed with curved surfaces and also have limitedsurface area in which to mount the solar cells. Consequently arrays ofcells are mounted on the curved surfaces but the variation of the angleof incidence of light on the different cells within the array on thecurved surface causes variation in the available optical power.Furthermore, cells in an array may be subjected to variable light levelsdue to shadowing by foreign objects such as trees and buildings betweenthe cell and the source of illumination.

Because of differences in optical illumination, cell temperatures mayvary within arrays causing some cells to be hotter than other cells.Arrays may be cooled partially by air flow or by the use of a coolingfluid in an illumination concentrator system. These mechanisms howevermay not provide uniform cooling to all cells.

The available power from each cell within an array will vary due to thevariations in illumination and temperature. In these cases, the maximumpower conditions of different cells within the array will differ at anyone point of time. Furthermore the maximum power conditions of somecells within the array will vary differently over time compared withothers. As well these variations are not predictable. In additionchanges to the maximum power conditions of cells can vary rapidlythereby requiring a relatively quick response time.

Currently maximum power tracking devices are directly electricallyconnected to an array of solar cells. A single maximum power trackingdevice is currently used to control the available power from an array ofbetween ten to several hundred cells.

OBJECT OF THE INVENTION

It is an object of the present invention to provide an alternate maximumpower tracking device that overcomes at least in part one or more of theabove mentioned problems or disadvantages.

SUMMARY OF THE INVENTION

The present invention arises from the realization that each cell at anyone particular time point will have a unique maximum power point definedby a specific cell voltage and specific current at which the cell willproduce its maximum available power. Furthermore the invention wasdeveloped from the realization that it is not possible for every cell inan array to operate at its maximum power point if the array is formed bythe direct electrical interconnection of cells. With this in mind andtaking advantage of recent advances in low voltage electronics, maximumpower tracking devices for very small groups of directly connected cellsor for single solar cells were developed to provide a solution tooptimizing the electrical power from the array.

In one aspect the present invention broadly resides in a system forproviding power from solar cells including one or more solar generatorswherein each of said solar generators has one to nine solar cells;

a maximum power tracker operatively associated with each solargenerator, each of said maximum power tracker includes a buck type DC/DCconverter without an output inductor, each of said maximum powertrackers are operatively connected in series with each other;

an inductor operatively connected to the series connected maximum powertrackers; and

means for providing electrical power from the inductor to load means,wherein each of said maximum power trackers is controlled so that theoperatively associated solar generator operates at its maximum powerpoint to extract maximum available power.

The maximum power tracker preferably includes an energy storagecapacitor and a control means for adjusting the buck type DC/DCconverter duty cycle so that a connected solar generator operates at itsmaximum power point.

Preferably the control means makes observations of solar generatorvoltage, and observations of the change in energy storage capacitorvoltage during the buck converter switch off time and observations ofthe duration of the buck converter switch off time to infer solargenerator power to adjust the buck converter duty cycle to extractmaximum power from the connected solar generator.

In one preferred embodiment, the switching operations of the DC/DCconverter are synchronized in frequency by the use of a synchronizingsignal.

Preferably each solar generator includes one solar cell. Preferably eachsolar generator includes one solar cell and each solar cell is connectedto its own dedicated maximum power tracker so that the tracker respondsto its connected solar cell.

Preferably the system uses a single inductor.

Load means includes devices that use or store the electrical power.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention can be more readily understood andput into practical effect, reference will now be made to theaccompanying drawings wherein:

FIG. 1 is a diagrammatic view of a simplified Buck type DC/DC converterwith solar generator and load;

FIG. 2 is a diagrammatic view of an alternative embodiment of asimplified Buck type DC/DC converter with solar generator and load;

FIG. 3 is a diagrammatic view of a solar generator with a Buck typeDC/DC converter without an inductor;

FIG. 4 is a diagrammatic view of an alternative embodiment of a solargenerator with a Buck type DC/DC converter without an inductor;

FIG. 5 is a diagrammatic view of the interconnection of a plurality ofBuck type DC/DC converters without inductors, corresponding plurality ofsolar generators, one inductor and a load; and

FIG. 6 is a diagrammatic view of a preferred embodiment of the singlecell MPPT converter;

FIG. 7 is a graphical representation of the control signals and gatesignals for MOSFETs;

FIG. 8 is a graphical representation of a no load 2 kHz waveforms, topMOSFET gate waveform; top MOSFET gate drive referred to ground, bottomMOSFET gate waveform to ground, output terminal to ground (from top tobottom);

FIG. 9 is a graphical representation of an unloaded 20 kHz waveforms,Traces top to bottom, output terminal, bottom MOSFET gate, top MOSFETgate, all referred to ground;

FIG. 10 is a graphical representation of a loaded 20 kHz waveforms,traces top to bottom, output terminal, bottom MOSFET gate, top MOSFETgate, all referred to ground;

FIG. 11 is a graphical representation of input voltage, current andpower at 10 kHz (from top to bottom);

FIG. 12 is a graphical representation of output current, voltage andpower at 10 kHz (from top to bottom);

FIG. 13 is a table of equipment for efficiency measurement; and

FIG. 14 is a table of converter efficiency at different frequencies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1 there is shown a simplified buck type DC/DCconverter 10 connected to a solar generator 11 and load 12. The solargenerator 11 can be a solar cell or several cells. The buck type DC/DCconverter 10 includes a capacitor 13 which serves as an energy storageelement, a controlled switching device 14, a diode or a controlleddevice acting as a synchronous rectifier 15 and an output inductor 16.An alternative arrangement for the buck type DC/DC converter 10 is shownin FIG. 2.

A buck type DC/DC converter can be controlled to operate the solargenerator at its maximum power point while producing an adjustable levelof output current. The solar generator and maximum power tracker will bereferred to as a solar generator/MPPT. Many solar generators/MPPT can beseries connected. Each DC/DC converter will then have an identicaloutput current but they can be individually controlled to allow eachsolar generator to operate at their maximum power point.

A conventional buck converter uses an output inductor to provide energystorage that is necessary for current filtering. An important feature ofthis invention is that the many inductors would normally be required,one for each solar generator/MPPT, and this can be replaced by a singleinductor which will perform the energy storage and filtering functionfor many series connected solar generator/MPPT. The MPPT device can beproduced as an inductor free device.

FIG. 3 shows an inductorless DC/DC buck converter with a solar generatorwhile FIG. 4 shows an alternate embodiment.

Many solar generators/MPPT devices that utilize inductor free DC/DC buckconverters can be series connected with a single inductor to supplypower to an electrical load. The series connection of the solargenerators/MPPT devices forces each inductorless DC/DC buck converter tosupply an identical output current. Each converter operates with aconstant current load.

The controlled switching device operates alternates between an open andclosed state. The average portion of time that the switch is closed isthe switch duty cycle. Closure of the controlled switching device causesthe load current to be supplied from the solar generator and the energystorage capacitor. When the controlled switch is open, the load currenttransfers to the diode or synchronous rectifier device while the solargenerator current replenishes the charge within energy storagecapacitor.

The duty cycle of the controlled switching device will determine theaverage current withdrawn from the energy storage capacitor. The energystorage capacitor will adjust its voltage in response to the differencein the current supplied by the solar generator and the current withdrawnto by the controlled switch. The switching device will be controlled bya device that adjusts the controlled switch duty cycle to maintain thesolar generator voltage at the maximum power point.

With respect to FIG. 5 there is shown a solar generator 20 connected toa capacitor 21, diode 22 and control switch 23. The capacitor 21, diode22 and control switch 23 forms the inductorless DC/DC converter 24.Several solar generators 20 are connected in series via their dedicatedinductorless DC/DC converters 24. Each solar generator 20 has its owninductorless DC/DC converters 24. After the last inductorless DC/DCconverters 24, there is an inductor 25 to filter the current prior toreaching the load 26. The inductor 25 can be smaller in terms ofmagnetic energy shortage measured as ½ LI² where L is the inductancevalue in Henry and I is the inductor current, in Amperes, than the totalcombined set of inductors that are normally used with each buck DC/DCconverter. The use of a smaller inductor and only one inductor reducescost and weight and increases the efficiency in providing maximum powerfrom the solar cells. In the preferred embodiment the solar generatorconsists of a solar generator which is a single high performance solarcell.

With reference to FIG. 6, there is shown a DC to DC converter 30 in theformed by MOSFETs Q1 and Q2 (31 and 32 respectively), and the energystorage capacitor 33. No filter inductor is required. In this preferredembodiment MOSFET Q1 (31) is a synchronous rectifier implementation ofthe diode device and MOSFET Q2 (32) is the buck converter controlledswitch element. In the preferred embodiment the output terminals of thesolar generator/MPPT device are the drain terminal of Q1, point X andthe junction of the source terminal of Q1 and the drain terminal of Q2,point Y.

The control element of the maximum power device is a microprocessor. Inthis preferred embodiment, an ultra-low power Texas Instruments MSP430microprocessor 34 which is capable of operation at a supply voltage of1.8V. This allows direct operation from a dual junction cell whichtypically produces 2V. If other cell types are used with lower cellvoltages, a power conditioning device may be required to develop ahigher voltage supply to allow the control element to be operated from asingle cell. For example, silicon cells typically produce 0.4V and avoltage boosting converter would be required to generate a voltage highenough to operate a microprocessor control element.

An alternate embodiment is possible where the solar generator/MPPTdevice output terminals are the junction of Q1 and Q2, point Y, and thesource of Q2. In this case Q1 is the controlled switch element and Q2 isthe diode element implemented as a synchronous rectifier.

The gate drive voltage for the MOSFETS Q1 and Q2 is derived by chargepump circuit. In the preferred implementation a multiple stage chargepump circuit formed by diodes D₁ to D₄, devices 35-38, and theirassociated capacitors 39-42.

The MOSFETS Q1 and Q2 are driven by a gate driver circuit. In thepreferred embodiment a comparator, 43, forms the driver circuit. As thiscircuit delivers a higher gate to source voltage to device Q2 than Q1,Q2 achieves a lower turn on resistance. In the preferred embodiment Q2is the controlled switching device as this arrangement minimizes powerlosses.

Resistors 44 and 45 form a voltage divider network which is used toperform voltage observations of solar generator voltage using a analogueto digital converter within the microprocessor 34. An important featureof the maximum power tracking method is the measurement of cell voltagemagnitude, the and measurement of the change in cell voltage duringperiods when the controlled switch, 32, is open and the measurement ofthe time that the controlled switch is open to infer cell power. Thismay be used as an input to a maximum power tracking method that willcontrol the DC-DC converter duty cycle to allow the solar generator tooperate at maximum power.

In order to secure high efficiency in the solar generator/MPPT, lowswitching frequencies are preferred. In the preferred embodimentswitching frequencies will be below 20 kHz. At very low switchingfrequencies the ripple voltage on capacitor C1 will increase. Thevoltage ripple will cause the cell to deviate from its maximum powerpoint. An optimum switching frequency range will exist. In the preferredembodiments the switching frequency will be adjusted to maximize theenergy delivered by the solar generator/MPPT.

A plurality of solar generator/MPPT may be configured within a largearray to switch at the same frequency and with a relative phaserelationship that provides improved cancellation of switching frequencyvoltage components in the output voltage waveforms of the solargenerator/MPPT combinations. This allows a smaller inductor to providefiltering of the load current. Such synchronization may be provided byauxiliary timing signals that are distributed within an array or byother means.

In some embodiments the solar generator/MPPT devices within an array maynot switch at the same frequency. The combined output voltage of largenumber of asynchronously switching series connected buck converters willfollow a binomial distribution. The average output voltage of the groupof n solar generator/MPPT devices, with an input voltage V_(in) and aduty cycle d, increases linearly with n while the switching ripple orthe distortion voltage, V_(dist), rises as √n.V _(dist) =V _(in)√{square root over (n(d−d ²))}  (1)

Likewise the average volt second area, A, for a shared filter inductorfollows an √n relationship.

$\begin{matrix}{A = {\sqrt{n}\frac{V_{in}}{f}\left( {d - d^{2}} \right)}} & (2)\end{matrix}$

In a non synchronized embodiment, a larger inductor is required than inan optimally synchronized embodiment. The required inductor is stillsignificantly smaller than the combined plurality of inductors thatwould be required for conventional buck converters.

A prototype converter was developed to first examine the conversionefficiency of the DC to DC converter stage and its suitability for usewith a dual junction single solar cell, with an approximate maximumpower point at 2V and 300 mA. For these tests the MSP340 was programmedto drive the charge pump circuitry and to operate the buck converterstage at a fixed 50% duty ratio. The experimental circuit is as in FIG.6. A fixed 2V input source voltage was applied and a load consisting ofa 2 500 μH inductor and a 1.6Ω resistor was applied. A dead-time of 0.8μS is inserted in each turn-on and turn-off transient to prevent MOSFETsshoot through conduction events.

As gate charging loss was a significant loss contributor, a range ofoperating frequencies was trialled. FIG. 7 shows the control waveformsat 20 kHz. The waveforms show the dead times between the top and bottomsignals at turn-on and turn-off. All waveforms in this figure are groundreferred. The measured no load loss in this condition was 6 mW which isapproximately twice the expected figure. The gate drive loss is fullydeveloped at no load and we may have additional loss in the charge pumpcircuitry. FIG. 8 shows gate waveforms at 2 kHz but a differentialmeasurement is made of V_(gs1) to show the lowering of the gate sourcevoltage to approximately 4V due to elevation of the source at the deviceturn-on.

The waveforms at 20 kHz without load are shown in FIG. 9. Note that theload connection is across terminals X and Y. The lower MOSFET has thehigher gate drive voltage and a lower R_(dson). FIG. 10 shows the loadedwaveforms. Note the conduction of the MOSFET inverse diodes in the deadtime as seen by the 2 μS wide peaks on the leading and trailing pulsetop edges on the top trace. The transfer of current to these diodesgenerates an additional conduction loss of 24 mW which reducesefficiency at higher frequencies.

Given circuit losses are around a few percentage points of rating,precise voltage and current measurements are needed if powermeasurements are used to determine efficiency. A complication is thatthe output is inductorless and both the output voltage and currentcontain significant switching frequency components. It is likely that asignificant amount of power is transferred to the combined R-L load atfrequencies other than DC.

In order to determine the efficiency of this converter, a new high endoscilloscope was used to measure the input and output power. Theinternal math function was employed to obtain the instantaneous powerfrom the current and voltage, the mean value of which indicates theaverage power. The current probe was carefully calibrated before eachcurrent measurement, to minimize measurement errors. FIG. 13 shows thedetails of the equipment used in a table format. FIGS. 11 and 12 showthe input and output voltages, current and power. The mean value ofmeasured power is displayed at the right column of the figures.

The efficiencies of the converter obtained are shown in a table in FIG.14. It is seen that the measured efficiency is slightly lower thanestimated especially at higher frequencies. One reason is the lossduring the dead-time. The on-state voltage drop of the diode is muchhigher than the MOSFET, and therefore reduces the efficiency of theconverter. At 10 kHz the dead time loss accounts for 12 mW of theobserved 30 mW. The results do confirm that the circuit is capable ofachieving high efficiencies especially if the switching frequency islow.

Variations

It will of course be realized that while the foregoing has been given byway of illustrative example of this invention, all such and othermodifications and variations thereto as would be apparent to personsskilled in the art are deemed to fall within the broad scope and ambitof this invention as is herein set forth.

Throughout the description and claims this specification the word“comprise” and variations of that word such as “comprises” and“comprising,” are not intended to exclude other additives, components,integers or steps.

What is claimed is:
 1. A system, comprising: a plurality of solar units,each solar unit of the solar units comprising: a solar generator havingat least one solar cell to generate electric power; only one pair ofoutput connections to provide output; and a DC/DC converter coupledbetween the solar generator and the pair of output connections toreceive electricity provided by the solar generator in entirety and toprovide the output via the pair of output connections, wherein the DC/DCconverter includes a switch, an energy storage capacitor coupled betweenthe solar generator and the switch, and a controller configured tocontrol the switch to operate the solar generator at a maximum powerpoint independent of other solar units of the plurality of solar units;an output inductor; and a set of wires configured to connect outputconnections of the solar units and the output inductor in series;wherein when the switch is turned on, the solar generator and the energystorage capacitor are connected in parallel and further connected in theseries connection of the solar units established by the set of wires;wherein when the switch is turned off, the solar generator and theenergy storage capacitor are electronically disconnected from the seriesconnection, and the DC/DC converter provides at least one path for theseries connection.
 2. The system of claim 1, wherein the at least onepath includes a diode.
 3. The system of claim 1, wherein the DC/DCconverter has no inductor.
 4. The system of claim 3, wherein the DC/DCconverter is a buck-type converter.
 5. A method, comprising: connectingoutputs of a plurality of solar units in series to form a seriesconnection, wherein each of the solar units comprising a solar generatorhaving at least one solar cell to generate electric power, and a DC/DCconverter coupled with the solar generator to receive the electric powergenerated by the solar generator in entirety and configured to outputthe electric power through the plurality of solar units connected inseries, wherein the DC/DC converter comprises a switch, and an energystorage capacitor coupled between the solar generator and the switch;wherein when the switch is turned on, the solar generator and the energystorage capacitor are connected in parallel for the series connection inthe plurality of solar units; wherein when the switch is turned off, thesolar generator and the energy storage capacitor are electronicallydisconnected from the series connection, and the DC/DC converterprovides at least one path for the series connection, and controllingthe switch of the DC/DC converter of each of the solar units to operatethe respective solar generator at a maximum power point independent ofother solar units of the plurality of solar units that are connected inseries.
 6. The method of claim 5, wherein each of the solar units has anenergy storage capacitor and a controller; and a duty cycle of the DC/DCconverter is adjusted via the controller to operate the solar generatorat the maximum power point, based on a voltage of the solar generator,and a voltage of the energy storage capacitor.
 7. The method of claim 5,further comprising: coupling an inductor with the plurality of solarunits in series to provide the electric power to a load.
 8. The methodof claim 7, wherein the DC/DC converter comprises a step down converter.9. The method of claim 8, wherein the inductor is a single inductorshared by DC/DC converters of the solar units connected in series. 10.The method of claim 7, further comprising: connecting the plurality ofsolar units to the load including devices that use or store the electricpower.
 11. The method of claim 5, wherein the at least one path includesa diode.
 12. The method of claim 11, wherein the DC/DC converter is abuck-type converter without an output inductor.
 13. A method,comprising: generating electricity in direct current (DC) using a solargenerator; and operating a first switch of a converter, coupled betweenthe solar generator and a series connection of solar generators,according to a duty cycle to allow the solar generator to supply a firstcurrent to the series connection of solar generators via the firstswitch of the converter, wherein the first switch is controlled tooperate the solar generator at a maximum power point independent ofother solar generators in the series connection of solar generators; theconverter is configured to allow a second current larger than the firstcurrent to flow through the series connection of solar generators, andthe converter includes an energy storage capacitor coupled in parallelwith the solar generator and coupled between the solar generator and thefirst switch; wherein when the first switch is turned on, the solargenerator provides the first current to the series connection of solargenerators, the energy storage capacitor provides a third current, andthe second current is equal to or larger than a sum of the first currentand the third current; and wherein, when the first switch is turned off,the solar generator and the energy storage capacitor are electronicallydisconnected from the series connection of solar generators, and theconverter provides at least one path for the series connection of solargenerators.
 14. The method of claim 13, wherein the solar generator hasat least one solar cell; and the converter has no inductor.
 15. Themethod of claim 14, wherein the converter is a buck-type converterwithout an output inductor.
 16. The method of claim 13, wherein when thefirst switch is turned on, an output voltage of the converter issubstantially equal to an output voltage of the solar generator.
 17. Themethod of claim 16, wherein the at least one path includes at least oneof: a diode, a second switch, wherein the converter turns off the secondswitch when the first switch is turned on, and a synchronous rectifier.18. The method of claim 13, wherein the converter further comprises acontroller to control the first switch according to the duty cycle. 19.The method of claim 18, wherein the controller is to further control thefirst switch according to at least one of: a phase shift, and asynchronizing signal.
 20. The method of claim 13, wherein the converteris a first converter and the solar generator is a first solar generator;and the series connection of solar generators further comprises a secondsolar generator and a second converter connected to the first converterin series, the second converter having at least a second switch, thesecond solar generator to provide electricity to the series connectionof solar generators via the second switch of the second converter, thesecond converter configured to allow the second current, larger than acurrent from the second solar generator, to flow through the seriesconnection of solar generators; and the method further comprises:operating a second switch of the second converter according to a dutycycle separately from the duty cycle according to which the first switchof the first converter is operated to separately maximize power outputfrom the first solar generator and the power output from the secondsolar generator.